Several types of converters are known for use in power supply systems, where there is a need to convert an AC power to a controlled DC power. The AC power will usually be supplied from an AC power source, such as the mains. The DC power is supplied to equipment such as telecommunication equipment, broad band data communication equipment (GSM/UMTS base stations etc), military equipment, medical equipment etc.
There are several requirements for such power supply systems. First of all, the efficiency should be high, i.e. the losses should be low. The power supply system described in WO 2009/028954 and WO 2009/058024 has an efficiency of ca 96% and is marketed and sold by Eltek Valere under the name FlatPack 2 HE. The power supply system provided as a unit for insertion into a rack. The unit has an height of 1 U (the standard height of one shelf in a rack, corresponding to 44.5 mm), a length of 328 mm and a width of 109 mm so that four such units may be provided next to each other in a 19″ rack. The unit may deliver a power of 2 kW or 3 kW at −48V DC.
An object of the next generation power supply is to provide a smaller unit having substantially the same power as the above power supply system and with a relatively high efficiency. More specifically, the new unit should be 1 U high. The length should be 220 mm so that the unit and the rack may be provided in a 30 cm power cabinet. In addition, the width should 72 mm in order to provide that six such units may be provided next to each other in a 19″ rack. The unit should be able to supply a power of 2-3000 W at −48V DC. Hence, the available volume for the components is reduced by approximately 55%.
Another object of the next generation power supply system is to reduce costs. One contribution to cost reduction is the reduced size. Another contribution to cost reduction is to use cheaper electronic components such as processor units etc.
However, such electronic components are simpler, and consequently, effort must be put into using the electronic components smarter. One way of achieving this is to reduce the number of calculations needed for controlling the power supply system.
The power supply system comprises a fan for blowing air through the unit. The fan is normally located on the front side of the unit and blows air out through the rear side of the unit. The increased power density (power per volume unit) of the unit makes it difficult to achieve a satisfying air flow through the unit.
U.S. Patent Application Publication No. 2004/170031 relates to a power supply system which is implemented without adding a filter circuit, by configuring an overvoltage protection circuit without a thyristor. A sub-loop control circuit is added to an AC/DC converter. The sub-loop control circuit is configured such that a photoreceptor side transistor of a photocoupler 26 has the collector terminal connected to the gate terminal of a MOS-FET 2 via a resistor 38, the emitter terminal is connected to the base terminal of a transistor 3, and the phototransmitter side of the photocoupler 26 is connected to an operational amplifier 39, resistors 40-43, and a Zener diode 44. In addition, in a DC-DC converter, a Zener diode 45 is connected across the input of the converter and the non-inverting input terminal of a comparator 33, with the anode of the Zener diode being connected to the non-inverting input terminal.
U.S. Patent Application Publication No. 2004/021992 relates to an overvoltage output protector being electrically connected to a constant-voltage switching power supply, which includes a switching transistor converting a DC voltage obtained by smoothing an AC voltage supplied from an AC power source into a cyclic pulse signal. In the overvoltage output protector, an overvoltage monitor indicates whether a potential of the cyclic pulse signal is at or exceeds a predetermined value. A deactivator turns off the switching transistor in a case where the overvoltage monitor indicates that the potential of the cyclic pulse signal is at or exceeds the predetermined value.
In some applications, there is a requirement for the safety integrity level (SIL) for the power supply unit. The safety integrity level is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. The International Electrotechnical Commission's (IEC) standard IEC EN 61508 defines SIL using requirements grouped into two broad categories: hardware safety integrity and systematic safety integrity. A device or system must meet the requirements for both categories to achieve a given SIL.
One example of such an application is equipment for the oil and gas industry, where the equipment needs a 24 V DC input, and where the function of the equipment is not guaranteed at voltages above 30 V CD, alternatively that a certain SIL level is not guaranteed at such voltages. Hence, one or more embodiments of the invention provide a power supply unit which have a considerably reduced risk of supplying power above 30 V DC, so that the risk of failure in the equipment itself is reduced. More specifically, on or more embodiments of the invention provide a power supply unit where the risk of supplying power above 30 V DC is categorized as SIL 3. Of course, the equipment may have a voltage limit being different than 30 V DC.
It should be noted that the SIL categorization often requires relative complex computations of probability analysis. The more complex the power supply unit is, the more complex the computations will be. Hence, one or more embodiments of the invention simplify the power supply unit in order to simplify the computation of SIL. One or more embodiments of the invention also provide an indication of the status, and hence the SIL status, of the power supply unit.